Such a device is known for example from U.S. Pat. No. 3,693,812. In this device, both rotors are rotated via a planetary gear box. From this planetary gear box, emerge: (1) a main input shaft; (2) a secondary input shaft; (3) a first output shaft, called rotary shaft hereafter; and (4) a second output shaft, called tilt controlling shaft hereafter. The main input shaft is rotated by a driving motor. A demultiplying mechanism connects the main input shaft to the rotary shaft. The latter penetrates into the support frame where it engages by means of a pinion with a toothed ring of the suspension rotor. The tilt controlling shaft itself also penetrates into the support frame, where it engages by means of a pinion with a toothed ring of the tilt controlling rotor. The planetary gear box further comprises: a horizontal annular toothed wheel, which engages at its external perimeter with a pinion of the rotary shaft; a solar wheel which is borne by the secondary input shaft; at least two satellite pinions, which engage with the internal perimeter of the annular toothed wheel and the solar wheel; and a satellite pinion carrier, which engages with a toothed wheel of the tilt controlling shaft. These gears are dimensioned so that both output shafts have the same speed of rotation when the secondary input shaft is not rotating. A control motor with a reversible direction of rotation is connected to the secondary input shaft of the planetary gear. By driving this control motor in a first direction, the chute is tilted in a first direction and by driving it in the reverse direction, the chute is tilted in the reverse direction. The speed of rotation of the control motor determines the tilt speed of the chute, regardless of the speed of rotation of the chute. By blocking the secondary input shaft by means of a brake, a strictly constant angle of inclination is provided for the rotating chute.
It will be noted that this planetary gear box is a key piece of equipment of the device for distributing bulk materials. This is a special design which accounts for a large portion of the price of the device. Further, in order to remain operational when the driving unit requires servicing or a major repair, a complete planetary gear box should be kept in reserve.